1. Field of the Invention
The present invention relates to printing presses. More particularly, the present invention relates to a printing press of the type including a liquid circulation system for providing constant flow of filtered liquid to a fountain pan on the press.
2. Description of the Prior Art
In a conventional printing press liquid circulating system, a cylindrical roller is partially immersed in the fountain pan and is rotated to pick up liquid from the pan and distribute a uniform film of liquid to a mating roller. Circulating the liquid in the fountain pan is desirable for several reasons. Circulation of the liquid maintains a minimum liquid temperature gradient across the pan. If the temperature gradient of the liquid across the pan is excessive the quality of the printing is adversely affected. Moreover, the circulation of the liquid prevents growth of algae. By circulating the liquid, foreign particles may be flushed from the pan and filtered from the liquid. In addition to providing a constant flow of water through the pan, a constant level of fluid in the pan is necessary to establish constant and desirable liquid feed rates to the rollers of the dampening system. Liquid which is being pumped into the pan, rises to the height of the standpipe and then drains through the standpipe to a return conduit.
The standpipe in the pan is connected via the liquid return conduit to a liquid reservoir wherein the liquid is stored before being recirculated to the fountain pan. In order to filter the liquid, that is, removed undesirable particles and other matter from the liquid, the liquid in the return conduit is passed through a filter prior to entering the reservoir. One particularly prevalent type of filter used in liquid circulating systems for printing presses is a cannister type filter, that is, a type of filter having a cylindrical perforated housing wrapped with a water permeable filter. The fluid is delivered into the interior of the cylinder and forced radially outwardly through the perforations in the housing. In order to provide the required pressure to force the liquid through the filter, an aspirator is connected in the return line upstream of the filter. The aspirator includes a motive flow passage through which liquid is pumped from the reservoir and back into the reservoir via the filter. The aspirator further includes an inlet port to which the return line from the fountain pan is connected. As liquid is pumped through the motive flow passage, liquid is drawn or suctioned from the return conduit and forced through the filter. Typically, the pump which provides for flow of liquid through the motive passage of the aspirator also provides for pumping of the liquid via a supply conduit back into the fountain pan on the press.
Although the above described circulation systems have functioned adequately for many years, one problem has been encountered during the use of the systems and is known in the printing art as "foaming." When the liquid is drawn from the fountain pan with the use of an aspirator, a negative pressure, that is, a pressure below atmospheric pressure, is induced in the return conduit. The negative pressure not only results in suctioning of the liquid through the return line, but also, results in the suctioning of air into the return line and through the remainder of the system. Thus, the air is suctioned into the aspirator and is mixed with the liquid being circulated through the aspirator. The introduction of air into the liquid causes foaming in the reservoir. In severe cases, foaming of the liquid in the reservoir may cause the reservoir to overflow. Moreover, the foam can be pumped by the circulating pump from the reservoir into the fountain pan. The presence of foam in the fountain pan has a noticeably adverse effect on the quality of printing. Foam in the water fountain pan of an offset printing press creates non uniform dampening on the dampening roller. This creates improper dampening of the non image area of the plate which in turn causes ink to plug into the non image area of the plate and hence the printed signature. Foam in the ink fountain of a flexagraphic press causes uneven ink distribution to the anilox fountain roller nip which results in uneven ink density on the printed signature.